1. Field of the Invention
The present invention relates to a viewfinder having an image display device, and also to a video camera utilizing the viewfinder.
2. Description of the Prior Art
A display device utilizing a liquid crystal display panel has been extensively studied and developed since it has some potential in that a further reduction in size and weight of the liquid crystal display device is possible as compared with a display device utilizing a cathode ray tube. In recent years. a liquid crystal display device using a twisted nematic (TN) mode in which the optical rotational polarization of liquid crystal molecules is utilized for an image display has been put to practical use and is currently used, for example, in a pocketable television set and a viewfinder in a video camera.
The details of a prior art viewfinder such as that disclosed, for example, in Japanese Laid-open Patent Publication No. 62-111233 is discussed below. It is to be noted that, in the instant specification, the term "viewfinder" is used to means an image display device of a type integrated with a light source such as, for example, a light emitting element, for illumination purpose.
In the prior art viewfinder, a tubular fluorescent lamp is utilized as a means for emitting light within the viewfinder. Where the liquid crystal display device has a display screen of about 1 inch in size, the fluorescent tube used therein is of a type having a diameter within the range of 2 to 5 mm. On the other hand, where the liquid crystal display device has a display screen of a size greater than 1 inch, more than one fluorescent tube is often used.
In either case, rays of light emitted from the light source travel in all directions radially outwardly therefrom and, in order to maximize the utilization of that portion of the rays of light which travel rearwardly from the light source, a generally concave reflector shade is disposed rearwardly of the light source to reflect the rays of light towards an .object to be illuminated that is positioned forwardly of the light source. A diffuser plate is also disposed between the light source and the TN liquid crystal panel for diffusing the incoming rays of light. A combination of the fluorescent light source with the diffuser plate constitutes a surface light source. The diffused rays of light, that is, the rays of light which have passed through the diffuser plate, are subsequently incident upon the liquid crystal display device to illuminate the latter so that an image displayed thereby can be viewable to the eye of a viewer looking into the viewfinder. As a matter of practice, the diffused rays of light travelling toward the liquid crystal display device have a cross section generally equal to or larger than the area of the surface of the display screen.
In the prior art viewfinder, a polarizing plate is disposed on each side of the TN liquid crystal panel forming the liquid crystal display device with respect to the direction of travel of the diffused rays of light. One of the polarizing plates that is positioned between the diffuser plate and the TN liquid crystal panel, hereinafter referred to as a polarizer, has a function of linearly polarizing the diffused rays of light, whereas the other polarizing plate that is positioned between the TN liquid crystal panel and the viewer's eye, hereinafter referred to as an analyzer, has a function of shielding the diffused rays of light according to the extent to which the incoming rays of light incident upon the TN liquid crystal panel are modulated. As is well known to those skilled in the art, the polarizer and the analyzer are are so arranged and so disposed that the direction of polarization of one of the polarizer and the analyzer can lie perpendicular to that of the other of the polarizer and the analyzer.
Thus, according to the prior art, the surface light source is constituted by at least one tubular fluorescent lamp and the diffuser plate. The diffused rays of light are, as they pass through the polarizer, converted into linearly polarized light, that is, light travelling in a wave formation along a straight path. This linearly polarized light is subsequently modulated by the TN liquid crystal display panel according to a video signal applied to the TN liquid crystal display panel and is then selectively intercepted and passed through the analyzer according to the modulation of the polarized light, so as to thereby effect a display of image information corresponding to the applied video signal. The use is often made of a magnifying lens disposed between the analyzer and the eye of the viewer to magnify the image information displayed.
The video camera, also known as a Camcorder, is generally required to be compact in size and light-weight in order for it to be portable and easy to handle. For this purpose. A liquid crystal display device is now often used as as a display element to be incorporated in the viewfinder. However, the liquid crystal display device currently designed for use in the viewfinder consumes a relatively large amount of electric power. By way of example, reports have been made that the viewfinder utilizing the prior art TN liquid crystal display device consumes about 1.1 watt, specifically about 0.1 watt by the display device and about 1.0 watt by the light source. In addition, in order for the video camera to have compact and portable features, a battery providing a source of electric power necessary to drive all electric circuits and motors used in the video camera has a limited capacity and, therefore, the higher the electric power consumption of the viewfinder, the shorter the length of time over which the video camera can be run continuously.
One of major causes of the consumption of a relatively high electric power by the prior art TN liquid crystal display device is discussed below. As is well known to those skilled in the art, the liquid display device utilizing the TN liquid crystal, that is, the TN liquid crystal display device, requires the use of the polarizing plates on respective sides of the TN liquid crystal panel with respect to the direction of travel of the light. These polarizing plates exhibit a composite light transmittance of about 30% which means that only about 30% of the incoming light passes through the TN liquid crystal display device including these polarizing plates. In other words, this 30% light transmittance means a 30% efficiency of utilization of the incoming light available to the TN liquid crystal display device.
In addition, an element that reduces the intensity of the incoming light available to the TN liquid crystal display device is the diffuser plate. As discussed above, a light box including at least one fluorescent lamp and the reflector shade is required to form a surface light source. This is accomplished by the use of the diffuser plate disposed between the TN liquid crystal display device and the fluorescent lamp. If the diffuser plate used is of a type having a relatively low light diffusing capability, a flickering of the fluorescent lamp tends to be noticeable, and therefore, the displayed image will be viewed in varying gradation accompanied by a reduction in image display quality. Therefore, it is a general practice to employ a diffuser plate having a relatively high light diffusing capability which, however, reduces the light transmittance of the diffuser plate to such an extent that the light source must have an increased light output to attain a required or desired luminance. This in turn brings about an increased electric power consumption.